For example, in an in-line screw type injection molding machine, the pressure of molten resin ahead (in front) of a head portion of a screw must be detected in order to control injection pressure or holding pressure during an injection step or in order to control back pressure during a metering step.
To this end, the in-line screw type injection machine is designed to detect the resin pressure with a load cell (strain detection sensor). There have been proposed various pressure detection methods (load detection methods) using load cells. According to one of the detection methods, a load cell unit is disposed between a direct-acting block moving forward/backward together with a screw and a direct-acting portion of a ball screw mechanism. A load (force) caused by resin pressure is detected by the load cell through the screw and the direct-acting block. In this method, however, the load cell unit is designed to be retained between the direct-acting block and the direct-acting portion of the ball screw mechanism for linearly driving the direct-acting block. It is therefore very troublesome to exchange the load cell unit, and it takes much time to do so.
There is also known a resin pressure detection method in which a load cell can be exchanged so easily as to enhance maintainability. According to the detection method, the load cell unit is mounted on an end portion of one of a plurality of connecting bars whose opposite ends are fixed to a head stock and a retention block respectively. The head stock retains a heating cylinder while the retention block is mounted with an injection servo motor. Thus, a load applied to the connecting bar is detected so that resin pressure can be detected. That is, in this detection method, for example, the load cell unit is disposed so that the load cell unit is loosely inserted into an end portion of a connecting bar on the head stock side while the load cell unit is placed between the head stock and a locknut fastened to the end portion of the connecting bar. Thus, the quantity of elastic deformation (quantity of strain) of an elastically deformable portion of the load cell unit is detected by a strain detection sensor pasted on the elastically deformable portion so that the load applied from resin to the connecting bar can be detected.
The connecting bars used in the background art have the same shapes with the same diameters. In the configuration where the load cell unit is mounted on an end portion of one of the connecting bars, the quantity of deformation (quantity of elastic deformation) which is the total sum of the quantity of deformation (quantity of elastic deformation) of the connecting bar where the load cell unit is mounted and the quantity of deformation (quantity of elastic deformation) of the load cell unit is therefore different from the quantity of deformation (quantity of elastic deformation) of any other connecting bar where the load cell unit is not mounted. For this reason, there arises a problem that the force applied to the injection mechanism becomes disproportionate with respect to the center of the screw axis. In order to avoid such a problem, a dummy load cell unit for producing the same quantity of deformation as that of the load cell unit may be mounted on any other connecting bar where the load cell unit is not mounted. In this manner, the quantities of deformation can be equalized to one another all over the connecting bars.
This is also applied to the configuration where a load cell unit is mounted on an end portion of one of connecting bars (tie bars) in a mold opening/closing system mechanism, that is, the configuration where a load cell unit is mounted on an end portion of one of a plurality of tie bars (connecting bars) whose opposite ends are fixed respectively to a fixed die plate mounted with a fixed mold and a tail stock mounted with a mold opening/closing servo motor. Also in this case, all the tie bars have the same shapes with the same diameters. Accordingly, the quantities of deformation cannot be equalized all over the connecting bars unless a dummy load cell unit for producing the same quantity of deformation as that of the load cell unit is mounted on any tie bar where the load cell unit is not mounted.
However, in the configuration using such a dummy load cell unit for producing the same quantity of deformation as that of the load cell unit, the number of parts increases and the number of man-hour for assembly also increases. Thus, there is a problem that the cost increases.